1. Field of the Invention
The present invention relates to an alignment layer for aligning molecules of a liquid crystalline material to a surface of a substrate. More specifically, the present invention relates to an alignment layer for aligning molecules of a liquid crystalline material to a surface of a substrate having a diffractive optical power region using a homogeneous alignment that is one of a tangential alignment, a piecewise tangential alignment, a perpendicular alignment, a piecewise perpendicular alignment, an oblique alignment, a piecewise oblique alignment, a continuous intra-zone alignment, a piecewise continuous intra-zone alignment, or a combination thereof.
2. Description of the Related Art
When using a layer of liquid crystalline material, it is necessary to align (i.e., anchor) molecules of the liquid crystalline material at the surface of a substrate. Most liquid crystalline materials are optically uniaxial and possess a single axis of symmetry with respect to their optical properties. This axis is known as the “director”. The liquid crystal director is a unit vector describing the average direction of orientation for liquid crystal molecules over a given region of the liquid crystal bulk. When the liquid crystalline material is unaligned, the orientation of the director varies throughout the bulk of the layer of material. Alignment of the molecules of the liquid crystalline material at the surface of the substrate generates a preferred alignment of the liquid crystalline material throughout the bulk of the layer such that the director is made to point, on average, in a single direction, called the alignment direction. When aligned, on average, in a single direction, optical waves traveling through the liquid crystalline material experience a known set of optical properties (e.g., index of refraction, phase retardation, or transmissivity). The orientation of the liquid crystalline molecules may then be altered from the preferred alignment direction in a predetermined manner (e.g., by means of applied electrical energy) to alter the optical properties that optical waves traveling through the liquid crystalline material experience.
While surface relief structures such as linear gratings and obliquely vacuum evaporated refractory oxides such as SiO have been used in the past to align liquid crystalline materials, their use in commercial applications has been mostly supplanted by layers of engineered alignment materials (i.e., alignment layers). An alignment layer may comprise a thin film of an organic, optically transparent material that is processed from solution. The alignment layer may induce preferential alignment of the liquid crystalline material. These engineered alignment materials may be used to create alignment layers that align molecules of the liquid crystalline material in an alignment direction that is mostly parallel to the substrate surface. Such an alignment is known as a homogeneous alignment. Alternatively, these materials may be used to create alignment layers that align molecules of the liquid crystalline material in an alignment direction that is mostly perpendicular to the substrate surface. Such an alignment is known as a homeotropic alignment.
Homogeneous alignment is typically achieved by mechanically rubbing or brushing a thin film of alignment material in a single direction using a pad or roller coated with a soft cloth such as, for example, synthetic velvet. By way of example only, the polyimide material SE-130 from Nissan Chemical Industries Ltd. is designed for this purpose. Homeotropic alignment is typically achieved with an alignment material that alters the surface energy of the substrate and causes one end of the molecules of liquid crystalline material, either the hydrophilic end or the hydrophobic end, to be strongly attracted or strongly repelled, thereby causing the molecules to stand on end. By way of example only, the polyimide material SE-1211 from Nissan Chemical Industries Ltd. is designed for this purpose. Such techniques are well known in the liquid crystal art.
More recently, photosensitive alignment materials have been developed for aligning molecules of liquid crystalline material. When exposed to linearly polarized UV radiation, these materials align the molecules in a direction that is either parallel or perpendicular to the direction of polarization of the UV radiation. By way of example only, the material ROP-103 from Rolic Technologies Ltd. is designed for this purpose.
However, the techniques for preferentially aligning molecules of liquid crystalline material with alignment materials have been optimized for flat-panel information displays built from mostly smooth, planar substrates where uniform, linear alignment over the surface of the substrates is required. Therefore, there is the need in the art for techniques for preferentially aligning molecules of liquid crystalline material with alignment materials in devices that comprise a substrate having non-planar fine features such as surface relief diffractive structures.